The present invention relates to an improved electronic cross-point array for telecommunication systems wherein the array is an xy matrix with semiconductor switches arranged at cross-points between x and y conductors that are connectable to each other by bringing the switches to an "on" state.
Cross-point arrays are in principle known previously, see e.g., the British Pat. No. 1,199,199, FIGS. 1 and 3. According to this British patent, transistors are used as switches. These transistors are in a known manner caused to assume a conducting state in and for the mentioned "on" state, while for the "off" position they are caused to assume a non-conducting state. However, the stated patent does not indicate that transistors do not have completely satisfactory properties owing to the fact that the ratio between the respective resistance-values in the "on" and "off" states is not sufficiently high, nor is the isolation between the required control circuit and the switch circuit sufficiently good.
The stated patent suggests the use of a field-effect transistor in a so called MOS structure. In view of the desire to manufacture the electronic cross-point arrays as an integrated circuit in a monolithic embodiment, i.e., with a considerable number of switches formed on one and the same semiconducting substrate, the indicated solution is not satisfactory.
What is desired is an electronic cross-point array or matrix consisting of a fully integrated monolithic circuit that can be made by the transistor manufacturing processes now in common use. For devices that include npn-transistors these processes start out from a substrate of p-doped medium-resistive silicon in the form of a mono-crystal. During the circuit manufacturing process first the n.sup.+ -doped so called sub-collector electrode is formed for each separate transistor, and subsequently a thin homogeneous n-doped epitaxial layer is applied to cover the entire sub-collector side of the substrate. The base and the emitter electrodes of each npn-transistor are subsequently diffused as p- and n.sup.+ -doped electrodes, resectively, into the epitaxial layer above the respective sub-collector. Around each transistor there usually is a p-doped isolation frame which is extended through the epitaxial layer to contact the substrate.
During the operation of the circuit, all three transistor electrodes must continuously by kept first and foremost at a positive voltage relative to the substrate. If the electronic cross-point array or matrix is based on transistors of the above-mentioned type used as switches, there has proved to be great difficulties in maintaining a high degree of isolation between the transistors and the substrate in general and between the substrate and the collector of the individual transistor in particular.